I bought and installed an Autometer electric FP gauge, and just followed their instructions.
The light for the gauge was connected to the headlight switch, tapping into the red/yellow wire. This was on my 2002 Mustang GT - obviously other years may have different wires. The only other wire (for the light) was simply grounded.
For the gauge itself, I tapped into Fuse # 20, which is a switched fuse. Then I had just one other wire again, and again it got the same treatment - it was grounded.
As WMBURNS indicates, you'll want to utilize your Schrader valve if you have one. Mine did, but rather than mounting directly onto the valve, I remotely mounted mine. Autometer recommends this method as it reduces the amount of vibration felt by the sending unit. See pic below.
The first pic is all of the fittings and such that I had to use as I was running a NOS system. I had to Tee off of the Schrader valve to feed both the NOS setup and the FP gauge. The second pic shows the hardware in mock up on the car (note the very well used AN fittings just for mock up purposes). You can see where I mounted the sending unit - on the shock tower.
Very sorry for being so vague. Its an 03 mustang and I was wondering which wire off the electronic regulator to tap into for the pressure signal. I know how to hook up the rest of the gauge.